1. Field of the Invention
The present invention relates to an oxide superconductor current lead to be used when supplying a current to a superconducting system used in an MRI, linear, SMES and the like, and to a method of manufacturing the same, and a superconducting system.
2. Description of the Related Art
A current lead, which is used when a large current is supplied to superconducting equipment such as a superconducting magnet, is for supplying a current of several hundreds to several thousands amperes to a cryogenic superconducting system from a power supply in a room temperature region. As the current lead, a copper wire with a low electrical resistance value is conventionally used. However, when the copper wire is used as a current lead, and a predetermined large current is passed through this, Joule heat is generated. Then, when a copper wire with a large wire diameter is used to reduce generation of Joule heat, heat penetration due to thermal conduction occurs to a side of the super conducting system via the copper wire having the large wire diameter, this time. As a result, power loss of a cryocooler and loss of a He gas as a refrigerant due to the heat penetration become serious. Thus, it is proposed in Patent Document 1 to include an oxide superconductor, which does not generate Joule heat even if a large current is passed through it, in the middle of this current lead.
[Patent Document 1]
Japanese Utility Model Laid-open No. 63-200307
Recently, development of superconductivity application equipment is advanced, and the level of the performance demanded of the oxide superconductor current leads becomes high, as a result of which, less heat penetration from the outside is demanded in addition to capability of passing a larger current, and less generation of Joule heat.
Here, the following factors are considered as the factors of Joule heat generation.
1) There is heat generation caused by contact resistance of joint portions of the oxide superconductor in the oxide superconductor current lead and metallic electrodes. The heat generation occurs because the oxide superconductor used for the oxide superconductor current lead is made of ceramics and has unfavorable joinability with metal, and thus the electric resistance (hereinafter, described as contact resistance) which cannot be ignored occurs to joint surfaces with the metallic electrodes (generally, a copper electrodes are used). Consequently, when a predetermined current is passed through the oxide superconductor current lead, heat is generated.
2) There is heat generation caused by resistance of the metallic electrodes themselves.
3) There is heat generation caused by contact resistance, following the transfer of a current at the joint portion of a mating conductor drawn out of the superconducting system side (hereinafter, described as the system side conductor) and the metallic electrode.
4) There is heat generation caused by contact resistance following the transfer of a current at the joint portion of a mating conductor drawn out of the power supply side (hereinafter, described as the power supply side conductor) and the metallic electrode.
Consequently, in order to reduce the value of the aforementioned contact resistance, interposing silver between the oxide superconductor and the copper electrodes in the form of the silver coat was tried first. Namely, paying attention to the fact that the contact resistance value between silver and the oxide superconductor is lower than the contact resistance value between copper and the oxide superconductor, silver foil is crimped to, a silver paste material is coated on, or silver is attached by thermal-spraying to the oxide superconductor, thereafter this is baked to be made silver coat, and this oxide superconductor with the silver coat and the copper electrodes are joined by using joining metal such as, for example, solder to form the oxide superconductor current lead.
However, as a result that a current passed through the current lead increases, generating Joule heat is not be ignorable with the current lead using the aforementioned oxide superconductor with the silver coat. Consequently, in order to reduce generation of Joule heat as passing a predetermined current though the current lead, the oxide superconductor is upsized, and the contact area with the copper electrodes is made larger.
As a result, though generation of Joule heat can be reduced, it becomes necessary to upsize the oxide superconductor to take the contact area of the oxide superconductor and the copper electrodes, and heat penetration from the high temperature side to the low temperature side is increased via the upsized oxide superconductor.
Thus, the oxide superconductor current lead as shown in, for example, FIG. 6 is considered.
In an oxide superconductor current lead 100 shown in FIG. 6, copper electrodes 120 as metallic electrodes are connected to both sides of a rare-earth based oxide superconductor 110 produced by the melting method, which is capable of passing a large current even with a small sectional area. Both end portions 112 of the rare-earth based oxide superconductor 110 have large sectional areas, but a central portion 111 has a small sectional area. Meanwhile, in the copper electrodes 120, contact portions 121 in contact with both the end portions 112 of the oxide superconductor are scraped to wrap up the both end portions 112, so that both of them can secure the large contact area.
This oxide conductor current lead 100 can restrain both the generation of Joule heat, and heat penetration from a high temperature side to a low temperature side even if a predetermined current is passed through it.
However, in the rare-earth based oxide superconductor produced by the melting method, which is suitable for the current lead among the oxide superconductors, it is difficult to produce a molded body with only a central portion being constricted to be slim as shown in FIG. 6. For this reason, in order to produce an oxide superconductor in such a shape, it is firstly necessary to produce a rare-earth based oxide superconductor in a rectangular parallelepiped shape of a size capable of securing a sufficient contact area with the metallic electrodes, and next, it is necessary to take a step of making a sectional area small by performing cutting work for the central portion in order to reduce heat penetration via the rare-earth based oxide superconductor. However, with this method, when a predetermined current value passed through the oxide superconductor current lead is large, a large-sized rare-earth based oxide superconductor is produced, and the large-sized rare-earth based oxide superconductor has to be cut large, thus reducing yields of the rare-earth based oxide superconductor and requiring a large number of man-hours. Further, the portions of the metallic electrodes are upsized, and therefore it is difficult to reduce the size of the entire oxide superconductor current lead.
Further, it has been considered that the contact resistance values at the joint portions of the metallic electrode and the system side conductor, and the metallic electrode and the power supply side conductor are reduced if the joint areas in the joint portions are made large. However, the problem that the reduction effect of the contact resistance value remains small even if the aforesaid joint area is only made large.
Thus, improvement in the joining method in the joint portions of the metallic electrodes, and the system side conductor and the power supply side conductor is tried by using different methods from the aforementioned silver coat interposal, and upsizing of the contact areas of the oxide superconductor and the copper electrodes, and various methods such as welding, brazing, crimping with various kinds of plating treatment being applied to the joint interfaces of both of them, and crimping with soft metal such as In flake at room temperature or the like being sandwiched between the joint interfaces of both of them have been carried out.
However, if the methods of heating the joint portions, such as welding and brazing are adopted for improvement in joining, thermal load is applied to the oxide superconductor in the current lead, as a result of which, the phenomenon that the oxide superconductor becomes rid of oxygen occurs, and the characteristics of the oxide superconductor are sometimes deteriorated. Further, even if the joint portions are welded or the like, variations in the contact resistance value in the joint interface of both of them cannot be restrained completely, and when a large current is passed, a drift current occurs to cause an increase in the contact resistance value.
When soft metal at room temperature, such as an. In flake or the like is sandwiched in the joint interface of the metallic electrode and the system side conductor and crimped or the like, variations in the contact resistance value in the joint interface of both of them cannot be restrained completely, and when a large current is passed, a drift current occurs to cause an increase in the contact resistance value.
Consequently, the object which the present invention is to attain is to provide an oxide superconductor current lead in which generation of Joule heat at joint spots with a system side conductor and a power supply side conductor is reduced, with use of an oxide superconductor with less heat penetration to a superconducting equipment system.